In applications such as telecommunications it is often necessary to detect certain special signals which may be present in the telecommunication signal stream. For example, in the DTMF telephone system, it is necessary to detect two tones to decode a key pressed on a telephone keypad. In the DTMF system, a tone detector must detect the presence of two of several possible tones to decode one of twelve telephone keys. The tone detector must reliably discriminate between tones and such sounds as the human voice and background noise. In the DTMF system, the frequencies at which the predetermined tones are expected are known with little uncertainty, making tone detection easier.
A tone is a sinusoidal wave at a specific frequency. One method of tone detection used in DTMF systems is to pass the input signal through a narrow-bandwidth bandpass filter, and then to measure the power in the bandpass-filtered signal. If a tone is present in the input signal, a high power level will be measured in the bandpass-filtered signal. The tone is detected by comparing the power in the bandpass-filtered signal with the power in another band of frequencies. This other band of frequencies is selected to exclude any possible tone frequency, such as the second harmonic of the highest expected tone frequency. If the power in the first bandpass-filtered signal exceeds the power in the other passband of frequencies by a predetermined threshold, then a tone is detected. If no tone is present, then the power measured in the bandpass-filtered signal will not exceed the power in the other band by the threshold. A change in the power ratio can also be used to determine the time at which the tone started or ended. A false-to-true transition in the power-ratio decision signifies a leading edge of the tone; a true-to-false transition in the power-ratio decision signifies a trailing edge of the tone.
However in other applications, a tone is expected at a frequency anywhere within a wide range. If the bandwidth of the bandpass filter is made wide enough to include any possible frequency for the tone, then significant amounts of other components such as white noise are also included, and the power-ratio decision may falsely indicate a tone when only white noise is present. Thus, the technique acceptable for the DTMF system described above fails to work properly for wideband tone detection, and different methods of tone detection must be employed.